Capillary suction in lizard skins

Surviving in the harsh desert environment is not an easy task. Yet three different lizards living in three different continents have developed the same solution. The Australian Thorny devil, Texas horned lizard and the Arabian toad-headed agama, have all the same special ability to collect water with the surfaces of their bodies. This evolutionary skin adaptation has not only arisen independently twice but three times!

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(A)Thorny devil (B)Texas horned lizard (C)Arabian toad-headed agama

This moisture or rain harvesting allow these animals to collect and distribute moisture over their entire body. By means of a capillary system on their skin, the water can then be transported to the mouth where it is ingested. This transport system is passive and thus requires no external energy.

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Thorny devil passive water flow

On the outer surface of the lizards’ scales, a microscopic film of water is held physically stable. This is due to the honeycomb-like micro ornamentation on the scales that render the surface superhydrophilic. Additionally, this surface also improves the condensation of air humidity considerably – 100% more compared to unstructured surfaces.

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Different sizes and morphologies of Texas horned lizard

In between the scales lies a complex capillary system consisting of micro-channels formed by partially overlapping scales. A narrow open on the superficial side of these interscalar channels form a semi-tubular capillary system on the whole body. This capillary network allows for an efficient passive water distribution and a directed transportation of water towards the mouth for ingestion.[1]

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Skin structure of Texas horned lizard

This phenomenon is based on geometric principles, involving periodic patterns interconnected capillary channels that narrow and widen. Based on these two principles, interconnections and asymmetry, scientists have developed a prototype in epoxy resin. This artificial abstraction of the surface could prevent liquid flow in one direction while sustaining in the other – thus achieving directional, passive liquid transport.[2]

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Principles of interconnections and asymmetry create a directional passive liquid transport

Consequently, based on these findings we can discern several strategies in our architectural adaptation in desert climate. A hydrophilic surface that allows moisture absorption from the air or from a nearby source. A capillary system that allows the collected moisture to be distributed throughout the system or towards a desired point. Ultimately this moisture must be used in some manner to help regulate the indoor temperature of a building; a possible solution is evaporative cooling.

A precedent that utilized these three solution is cool brick. A 3D printed porous ceramic bricks that absorb water like a sponge. This porous material satisfies both hydrophilicity and to a certain degree, water distribution, though not directed. The 3d lattice structure of cool bricks allows air to pass through the wall and lower the temperature using evaporative cooling.

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Evaporative cooling airflow diagram of cool brick

There are multiple ways to incorporate these principles into a natural building structure. This example below proposed a system integrated in a court garden using a mashrabiya inspired mesh-structure. This structure will act as a water container, drawing water from fountains through passive water flow principle and let water evaporate to cool down the interior space.

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Concept collage by Polina Moroz, 2016

Another approach is to integrate these systems through the use of tiles. Individual tiles with hydrophilic and water-distributive properties can form a wall covering system that have the ability to modulate the internal climate of a building. With the principles of water distribution and evaporative cooling, this tile-system becomes a building’s cool inner skin. These wettable and hydrophilic internal cladding can take advantage of existing traditional pattern to give it a historical context. Vernacular traditional facade ornaments can therefore be great source of inspiration in the development of these tiles.

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Concept of tiling system with passive water flow principle

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The tiling system forms a cool inner envelope that modulates the internal climate

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Traditional façade ornament

The design process of these tiles will involve working with 2D surfaces on a 3D mass. The capillary pattern that distributes the water has to be configured in 2D. Later on, this pattern will be projected onto an irregularly shaped 3D mass, creating grooves and channels through which water can flow through.

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Wettable internal cladding

The design of the 2D pattern and the 3D mass will be determined by many factors, all influencing each other. The choice of material will affect the design process, yielding different arrangements and properties. Using parametric/scripted design will also give rise to multiple optimizing solutions e.g. the maximization of surface area on both macro and micro scale. Can the concept of fractal be useful? The fabrication and manufacturing process will have to look into both modern and traditional/local practice to find the most desirable solution in regard to practicality, accessibility and cultural context. Will it be an additive or subtractive process or will it be a combination of both? One approach is to 3D-print a model and then laser-cut it with microscopic precision. Or is it more feasible with natural growth in form of crystallization?tilblackwhite.pngU_95_387374024997_Screenshot20150702at4.50.36PM.jpglaser_engraving.jpg

Thanks to the scientific heavy work already done by scientists and engineers, the principles surrounding the water collecting ability of the lizards’ skins are systematically mapped and simplified. With the artificial abstraction of the capillary systems, the next step is to implement these discoveries onto an architectural structure that encompass all the principles to allow a comfortable indoor climate. Many challenges and investigations remain and a functioning system may require additional principles and the integration of these. The translation of the lizard’s skin into a building structure will also touch on the questions of scale. A building’s skin is many magnitudes bigger than a lizard’s skin. Will water distribution even be able to overcome gravity and rise more than a couple of centimeters?

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Concept collage of a structure utilizing water distribution principles

Reference

  1. The Royal Society publishing, “Directional, passive liquid transport: the Texas horned lizard as a model for a biomimetic ‘liquid diode’” Philipp ComannsGerda BuchbergerAndreas BuchsbaumRichard BaumgartnerAlexander KoglerSiegfried BauerWerner Baumgartner,
  2. Moisture harvesting and water transport through specialized micro-structures on the integument of lizards “ Philipp Comanns1Christian Effertz2Florian Hischen1Konrad Staudt1Wolfgang Böhme3 and Werner Baumgartner. © 2011 Comanns et al; licensee Beilstein-Institut

 

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